Store IsolateHandle data in its own allocation

[coolreader18]

Currently, IsolateHandle is unsound - it mutates IsolateAnnex.isolate through an immutable reference with no UnsafeCell. A much cleaner solution is to store the data for IsolateHandle in its own allocation, since that also means an extant IsolateHandle won't prevent the other data in IsolateAnnex from being deallocated. I originally was going to store the pointer to IsolateHandleInner in its own isolate data slot, but realized it was also possible to just still store it in IsolateAnnex for simplicity.

[coolreader18]

I suppose another option is to not have any extra Arc indirection and just store a Weak<IsolateAnnex> in IsolateHandle, but then you don't get the unsynchronized access of IsolateHandle::get_isolate_ptr for Globals.